Prosthesis and method for using prosthesis to facilitate deep knee flexion

ABSTRACT

The present invention provides a femoral prosthesis for a femur which can enable or allow deep knee flexion without creating excessive tension in the ligamentous structure of the knee. The femoral prosthesis includes an internal non-articulating surface, an external articulating surface, a medial condyle and a lateral condyle. The height of the medial condyle is less than the height of the lateral condyle. A proximal-posterior tip of the medial condyle is rounded and is shifted inwards relative to the native level of the proximal-posterior region of the femur bone to facilitate knee flexion. A method of mounting a femoral prosthesis on a femur is also described.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 14/790,694 filedJul. 2, 2015, which is a continuation of U.S. Ser. No. 13/886,512 filedMay 3, 2013, now U.S. Pat. No. 9,084,679, which claims the benefit of,and incorporates herein by reference, U.S. Provisional PatentApplication Ser. No. 61/642,073, filed on May 3, 2012.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not Applicable.

BACKGROUND OF THE INVENTION

The present invention relates to systems and methods for prostheticsand, more particularly, to systems and method for femoral prosthesis forknee replacement implants and methods of using femoral prosthesis forknee replacement.

Knee replacement procedures, such as total knee arthroplasty (“TKA”) area highly successful surgical treatment option for severe knee jointdiseases, such as osteoarthritis and rheumatoid arthritis. However,several biomechanical and clinical studies have shown that current TKAprostheses do not fully restore the normal function of the knee.Typically, the active range of knee flexion following TKA is less than115 degrees, whereas the healthy knee is capable of knee flexion up to160 degrees (Sultan P G et al., Optimizing Flexion After Total KneeArthroplasty: Advances in Prosthetic Design, Clin Orthop Relat Res.2003:167-173; Dennis D A et al., Factors Affecting Flexion After TotalKnee Arthroplasty, Clin Orthop Relat Res. 2007; 464:53-60). Even whencompared to age-matched control population, TKA patients have been shownto have reduced knee flexion range. For example, Nutton et al. showedthat the average active flexion range for TKA patients was 110 degrees(average age 71 years), whereas the age matched control population(average age 69 years) had average active knee flexion range of 134degrees (Nutton R W et al., A Prospective Randomised Double-blind Studyof Functional Outcome and Range of Flexion Following Total KneeReplacement with the NexGen Standard and High Flexion Components, J BoneJoint Surg Br. 2008; 90(1):37-42). Similarly, Noble et al. found thatTKA patients had more functional impairment compared to normal subjectsof similar age, particularly with regards to deep knee flexionactivities involving kneeling and squatting (Noble P C, Gordon M J etal., Does Total Knee Replacement Restore Normal Knee Function, ClinOrthop Relat Res. 2005; 431:157-165).

These types of activities are particularly important for occupationssuch as roof tiling, leisure activities such gardening, and patientsfrom certain ethnic and cultural backgrounds (e.g. Japan, India, China,and other Asian and Mideastern countries) (Sultan P G et al., OptimizingFlexion After Total knee Arthroplasty: Advances in Prosthetic Design,Clin Orthop Relat Res. 2003:167-173; Dennis D A et al., FactorsAffecting Flexion After Total Knee Arthroplasty, Clin Orthop Relat Res.2007; 464:53-60). Increased range of knee flexion is also important formeeting the higher demands of younger patients who are increasinglyreceiving these prostheses. In recognition of these needs, several majororthopaedic companies have put forth so-called high-flexion TKAprostheses (e.g. Sigma CR150 High Flex from Depuy Inc, and NexGenCR-Flex from Zimmer Inc). However, recent studies have shown that theseHigh-Flexion TKA prostheses offer no advantage over standard prostheseswith regards to increasing the range of knee flexion (Gandhi R et al.,High-flexion Implants in Primary Total Knee Arthroplasty: aMeta-analysis, Knee, 2009; 16(1):14-7).

One of the causes for restricted knee flexion range with current TKAprostheses is that they create excessive tension in the ligamentousstructure of the knee in deep flexion. Deep flexion of the knee isgenerally described as flexion greater than about 115 degrees flexion.This may result from overstuffing of the flexion joint space by theprosthetic components (Varadarajan K M et al., Tibiofemoral Joint SpaceMeasured During Weight-Bearing Knee Flexion Increases Following TKA,Proceedings of 56th Annual Meeting Orthop Res Soc, New Orleans, La.,March 2010). In a series of inter-related studies Jeffcote et al.,Nicholls et al. and Kuster et al. used a combination of miniature forceplates and spring loaded rods to understand the pattern of soft tissuetension in the native knee and in the knee after implantation ofcontemporary (prior art) TKA prosthesis (Jeffcote B et al., TheVariation in Medial and Lateral Collateral Ligament Strain andTibiofemoral Forces Following Changes in the Flexion and Extension Gapsin Total Knee Replacement, A Laboratory Experiment Using Cadaver Knees,J Bone Joint Surg Br. 2007 November; 89(11):1528-33; Nicholls R L etal., Tibiofemoral Force Following Total Knee Arthroplasty: Comparison ofFour Prosthesis Designs in Vitro. J Orthop Res. 2007 November;25(11):1506-12; Kuster M S et al, Assessment of Isometricity Before andAfter Total Knee Arthroplasty: a Cadaver Study. Knee. 2009 October;16(5):352-7). In these studies the soft tissue tension in the nativeknees was found to be relatively low and uniform in the 15-90 degreesflexion range, such as further illustrated in the graphs of FIGS. 1A and1B. The native knees were also slightly tighter in full extension (0degrees flexion), and gradually tightened from 90 degrees to 150 degreesflexion. Here, tightness of the joint implies increased soft tissuetension. Following implantation of contemporary TKA prosthesis, theknees showed a similar pattern of soft tissue tension as the nativeknees in the 0-90 degrees flexion range, such as indicated in FIGS. 1Aand 1B. However, beyond 90 degrees flexion the TKA knees showed a rapidincrease in soft tissue tension. This was seen for various contemporaryTKA designs particularly those involving the retention of the posteriorcruciate ligament (Nicholls R L et al., Tibiofemoral Force FollowingTotal Knee Arthroplasty: Comparison of Four Prosthesis Designs in Vitro,J Orthop Res. 2007 November; 25(11): 1506-12). This excessive tighteningof the soft tissues of the knee in deep flexion could contribute to therestricted range of knee flexion following implantation of contemporaryTKA prosthesis.

The above studies were done on cadaver knees under non-weightbearingconditions. However, these findings are also supported by a more recentstudy wherein the tibiofemoral joint space in knees of TKA patients wasmeasured during a weight-bearing activity, and compared to tibiofemoraljoint space in healthy knees of normal subjects (Varadarajan K M, YueB., Moynihan A L, Seon J K, Freiberg A A, Rubash H E, Li G.,Tibiofemoral Joint Space Measured During Weight-Bearing Knee FlexionIncreases Following TKA. Proceedings of 56th Annual Meeting Orthop Res.Soc., New Orleans, L A, March 2010). This study found that the TKA kneesshowed increased tibiofemoral joint space compared to healthy knees forflexion above 90 degrees, such as further illustrated in the graph ofFIG. 2. Herein, the tibiofermoral joint space was defined as thedistance between a point on the femur bone and a point on the tibia bonemeasured in the proximal-distal direction. Most TKA patients in thisstudy could not bend their knees beyond 110 degrees flexion. Theincreased tibiofemoral joint space in flexion may be lead to increasedsoft tissue tension, which may contribute to restricted range of kneeflexion with prior art TKA prosthesis.

Therefore, it would be desirable to have a system and method that canprovide deep knee flexion without creating excessive tension in theligamentous structure of the knee, and thereby to restore native anatomyand function of the knee.

SUMMARY OF THE INVENTION

The foregoing needs are met by providing a femoral prosthesis, which canfacilitate or allow deep knee flexion without creating excessive tensionin the ligamentous structure of the knee. The design of this prosthesiscan prevent over tensioning of the ligaments in deep flexion, therebyfacilitating increased range of motion.

In one aspect, the present disclosure provides a prosthesis is disclosedfor a femur configured to control over-stretching of soft tissue in deepknee flexion. The prosthesis includes an internal non-articulatingsurface and an external articulating surface. The external articulatingsurface includes a distal articulating surface, a medial posteriorsurface having a first medial end and a second medial end, and a lateralposterior surface having a first lateral end and a second lateral end.The first medial end and the first lateral end are connected to thedistal articulating surface. The prosthesis also includes a medialcondyle extending from the first medial end to the second medial end andformed between the medial posterior surface and the internalnon-articulating surface and a lateral condyle extending from the firstlateral end to the second lateral end and formed between the lateralposterior surface and the internal non-articulating surface. A firstheight of the medial condyle is measured from a line tangent to thedistal articulating surface to the second medial end and is less than asecond height of the lateral condyle measured from the line tangent tothe distal articulating surface to the second lateral end.

In another aspect, the present disclosure provides a method is providedfor mounting a femoral prosthesis on a distal end of a femur associatedwith a knee and a leg to control over-stretching soft tissue in deepknee flexion. The method includes providing a femoral prosthesis with adistal articulating surface and a lateral posterior surface, removing aportion of a proximal-posterior bone of the femur, and mounting thefemoral prosthesis on the femur. The femoral prosthesis is mounted tothe femur such that the distal articulating surface is tangent to acartilage surface on a lateral side of the knee, the distal articulatingsurface is perpendicular to a mechanical axis of the femur, and thelateral posterior surface is tangent to a posterior cartilage surface onthe lateral side of the knee.

These and other features, aspects, and advantages of the presentinvention will become better understood upon consideration of thefollowing detailed description, drawings and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following drawings, like reference numerals will be used to referto like parts from Figure to Figure.

FIG. 1A provides a plot of displacement of a floating rod in the intactand TKA knees as a function of knee flexion angle. This is used tocharacterize soft tissue tension pattern, where higher displacementvalues correspond to greater soft tissue tension, in accordance withprior art.

FIG. 1B shows a plot of tibiofemoral joint compression force as afunction of flexion angle in a knee with prior art TKA prosthesisfollowing standard joint balancing procedure (BG=balanced gap), andexperimental modifications involving increasing the extension gap (EG)and flexion gap (FG) by 2 mm. This is used to characterize soft tissuetension pattern, where higher force values correspond to greater softtissue tension and generally indicate greater tightness, in accordancewith prior art.

FIG. 2 provides a plot of tibiofemoral joint space measured in knees ofpatients with prior art TKA prosthesis post-surgery minus thecorresponding joint space in the knees of healthy subjects, as afunction of knee flexion angle, where above 75 degrees flexion, thetibiofemoral joint space in the TKA knees increased rapidly compared tothe corresponding values in healthy knees, in accordance with prior art.

FIG. 3A provides a front elevational view illustrating a native femur,indicating the alignment/positioning of a femoral prosthesis on thenative femur following standard surgical procedure which aligns thefemoral prosthesis perpendicular to the femoral mechanical axis andtangent to the lateral femoral cartilage, in accordance with prior art.

FIG. 3B provides a top elevational view illustrating the procedure ofFIG. 3A, in which the posterior surface of the femoral prosthesis isaligned tangent to the lateral cartilage, with approximately 3 degreesof rotation (α) relative to a line tangent to the medial and lateralfemoral cartilage surface.

FIG. 4A provides a side view of a prior art femoral prosthesis,displaying the profiles of the medial and lateral condyles of thefemoral prosthesis. Circles fit to the distal-posterior profile of themedial/lateral condyle in the side view are defined as medial/lateralcondyle circles. Centers of these circles are defined as medial/lateralcondyle centers.

FIG. 4B provides a side view of a prior art TKA femoral prosthesis,displaying the profiles of the medial and lateral condyles. Herein themedial and lateral femoral condyles of the prior art prosthesis haveequal heights (h′_(m)=h′_(l)).

FIG. 4C provides sagittal sectional views of the prior art femoralprosthesis of FIG. 4A taken parallel to the femoral mechanical axisdisplaying both the medial and lateral condyles of the femoralprosthesis, in which the medial and lateral condyles of the femoralprosthesis have equal heights (h′_(m)=h′_(l)), and the medial side ofthe femoral prosthesis matches the level of the femoral bone in thedistal and posterior regions, but is proud of the bone in theproximal-posterior region i.e. is shifted outwards relative to thefemoral bone surface.

FIG. 5A shows a side view of another prior art femoral prosthesis,displaying the profiles of the medial and lateral condyles. Herein theheight of the medial femoral condyle of the prior art femoral prosthesisis greater than the height of the lateral femoral condyle(h″_(m)>h″_(l)).

FIG. 5B shows sagittal sectional views of the prior art femoralprosthesis of FIG. 5A taken parallel to the femoral mechanical axisdisplaying both the medial and lateral condyles of the femoralprosthesis, in which the height of the medial condyle of the femoralprosthesis is greater than of the lateral condyle (h″_(m)>h″_(l)).Herein, the medial side of the femoral prosthesis matches the level ofthe femoral bone in the distal and posterior regions, but is proud ofthe bone in the proximal-posterior region i.e. is shifted outwardsrelative to the femoral bone surface.

FIG. 6A shows a side view of one embodiment of the femoral prosthesis ofthe present invention, displaying the profiles of the medial and lateralcondyles. Herein the height of the medial femoral condyle of the novelfemoral prosthesis is less than the height of the lateral femoralcondyle (hm<hl), and the proximal-posterior articulating surface of themedial femoral condyle is shifted inwards relative to theproximal-posterior portion of the lateral femoral condyle.

FIG. 6B shows sagittal sectional views of one embodiment of the femoralprosthesis of the present invention taken parallel to the femoralmechanical axis displaying both the medial and lateral femoral condylesin which on the medial side, the prosthesis matches the level of thefemoral bone in the distal, posterior, and proximal-posterior regions,and the proximal most tip/end of the medial condyle is rounded andlowered below the level of the native femur bone i.e. shifted inwardsrelative to the native femur bone.

FIG. 7A shows a posterior view of a prior art TKA femoral prosthesis inwhich the medial and lateral condyle heights are equal.

FIG. 7B provides a posterior view of another prior art TKA femoralprosthesis in which the medial condyle height is greater than thelateral condyle height.

FIG. 7C provides a posterior view of a femoral prosthesis in accordancewith the present disclosure in which the medial condyle height is lowerthan the lateral condyle height.

FIG. 8A shows medial side views of a femoral prosthesis in accordancewith the present disclosure in varying degrees of flexion, in which thefemoral prosthesis allows for uniform ligament tension throughout thefull range of knee motion including deep flexion, with the profile of astandard femoral prosthesis of FIG. 4B being shown for comparativepurposes.

FIG. 8B provides side views displaying the difference between a standardfemoral prosthesis of FIG. 4B, and a femoral prosthesis in accordancewith the present invention in varying degrees of deep flexion, in whichthe standard femoral prosthesis results in over tensioning of theligaments in deep flexion.

FIG. 9 provides a graph showing tension/force in the posterior cruciateligament during a simulated deep knee bend activity with prior artfemoral prosthesis of FIG. 4B and embodiment of the femoral prosthesisof FIG. 6A.

DETAILED DESCRIPTION OF THE INVENTION

Certain exemplary embodiments will now be described to provide anoverall understanding of the principles of the structure, function,manufacture, and use of the devices and methods disclosed herein. One ormore examples of these embodiments are illustrated in the accompanyingdrawings. Those skilled in the art will understand that the devices andmethods specifically described herein and illustrated in theaccompanying drawings are non-limiting exemplary embodiments and thatthe scope of the present invention is defined solely by the claims. Thefeatures illustrated or described in connection with one exemplaryembodiment may be combined with the features of other embodiments. Suchmodifications and variations are intended to be included within thescope of the present invention. The definitions of a terms used todescribe the present invention are provided below.

The term “native” is used herein to imply natural or naturally occurringin the body. Examples of native structures include musculoskeletalstructures such as the femoral bone (or femur), tibial bone (or tibia),tendon, muscle, ligament, and the like.

The terms “articulate” and “articulating”, are used herein to indicatethe possibility of relative motion at a surface. For prostheticcomponents such relative motion is intended or part of the designintent. For example, “component A articulates with component B”,indicates that relative motion can occur between component A andcomponent B at the mating surface/s or interface/s.

The term “articular surface” as used herein refers to a portion of anative musculoskeletal structure or a prosthetic component whererelative motion (or articulation) can occur in relation to anothernative structure or prosthetic component.

The term “condyle circle” as used herein is a circle that best fits orapproximates the distal-posterior profile of a femoral condyle, such asa medial or lateral femoral condyle, as seen in a side view. The centerof a condyle circle as seen in a side view is defined as the “condylecenter” (FIG. 4A).

The term “inwards” as used herein means toward the inside or toward theinterior, such as toward or closer to a center. For example, a portionof an outer surface of a medial or lateral condyle of a femoralprosthesis may be shaped, designed or contoured to move or shift thesurface inwards toward the condyle center.

The term “outwards” as used herein means away from the inside or towardthe exterior, such as away from a center. For example, a portion of anouter surface of a medial or lateral condyle of a femoral prosthesis maybe shaped, designed or contoured to move or shift the surface outwards,away from the condyle center and towards the exterior.

The term “condyle height” as defined and used herein is the distancebetween a line or plane tangent to the distal articulating surface ofthe condyle and another line or plane tangent to a proximal end of thecondyle, for example as may be additionally understood from FIGS. 6A and7C.

Referring to FIGS. 3A and 3B, the orientation or positioning of afemoral prosthesis on a distal end of a femur bone following standardTKA surgical procedure is shown. A femoral prosthesis is generallymounted such that the distal surface 16 of the prosthesis will beapproximately perpendicular to the mechanical axis 18 of the femur 12.In order to mount the femoral prosthesis on the femur 12, thecartilaginous surface 20 of the side with relatively normal anatomy istypically used as reference. As the majority of the surgical cases(greater than about two-thirds) involve disease of the medial side 22 ofthe knee, the lateral side 24 of the knee is typically used as thereference.

Referring to FIG. 3A, a femoral prosthesis is mounted such that on thelateral side 24 of the knee, a distal surface 16 of the prosthesis isoriented on a line 21 tangent to the original cartilage surface 20 onthe lateral side 24 of the knee. The distal surface 16 of the prosthesisis then oriented to be perpendicular to the mechanical axis 18 of thefemur 12. This results in the distal surface 16 of the femoralprosthesis approximately matching the level of the native femoral bone12 on the medial side 22. The angle α created by the rotation betweenthe initial orientation of the distal surface 16 along line 21 to itsfinal orientation to be perpendicular to the mechanical axis 18 of thefemur 12 is approximately equal to three degrees, however, it can beappreciated that this angle α may vary. For example, α maybe in therange of 1 degree to 10 degrees, 3 degrees to 7 degrees, 4 degrees to 6degrees, and the like.

Turning now to FIG. 3B, the orientation for the posterior surface 26 ofa prosthesis is shown. The posterior surface 26 is positioned to matchthe posterior cartilage surface 28 on the lateral side 24 of the knee,as referenced by line 23. Similar to the rotation described above, theposterior surface 26 is rotated such that the posterior surface 26approximately matches the level of the native femoral bone 12 on themedial side 22 of the knee. The angle β created by the rotation betweenthe initial orientation of the posterior surface 26 along line 23 to itsfinal orientation is approximately equal to three degrees, however, itcan be appreciated that this angle β may vary. For example, β maybe inthe range of 1 degree to 10 degrees, 3 degrees to 7 degrees, 4 degreesto 6 degrees, and the like.

The orientation of the distal and posterior surfaces 16, 26 of a femoralprosthesis as described achieves a symmetric joint space to enable equaltensioning of the medial and lateral collateral ligaments in the 0degrees to 90 degrees flexion range in the distal 29 and posterior 31regions of the femur 12. As such, components for a standard TKA femoralprosthesis 10 are designed to allow the distal surface 16 to beperpendicular to the mechanical axis 18 while achieving equalmedial/lateral ligament tension from 0 to 90 degrees flexion. However,this does not account for ligamentous function beyond 90 degreesflexion. Therefore, when the above surgical procedure is employed with astandard prosthesis 10, prior art prostheses such as shown in figuresFIGS. 4B and 5A, the medial posterior surface 38 of the prosthesis 10does not match the femoral bone surface 13 at flexion angles greaterthan 90 degrees, such as further illustrated in FIGS. 4B, 4C, 5A and 5B.This provides that when the knee is in a state of flexion beyond 90degrees, the traditional femoral prostheses do not match the level ofthe native femoral bone surface 13 in the proximal-posterior region 27on the medial side. This can lead to excessive ligament tension in theknee, particularly in medial structures of the knee such as the medialcollateral ligament, and structures that attach to the medial femoralcondyle, such as the posterior cruciate ligament.

Turning now to FIGS. 6A and 6B, a femoral prosthesis design 100 inaccordance with the present disclosure is shown. The prosthesis 100includes an internal non-articulating surface 132. The internalnon-articulating surface 132 is connected to the resected distal end ofa femur 12 during surgery, as one of ordinary skill in the art of TKA isfamiliar, such as with bone cement. The femoral prosthesis design 100also includes an external articulating surface 134 that engages andarticulates with the tibial component (not shown) of the TKA prosthesis.The external articulating surface 134 includes a distal articulatingsurface 136 and a medial posterior surface 138 that has a first medialend 140 and a second medial end 142. The external articulating surface134 also includes a lateral posterior surface 144 that has a firstlateral end 146 and a second lateral end 148. The first medial end 140and the first lateral end 146 are connected to the distal articulatingsurface 136. A medial condyle 150 extends from the first medial end 140to the second medial end 142 and is formed between the medial posteriorsurface 138 and the internal non-articulating surface 132. The medialcondyle 150 includes a proximal-posterior tip or end 151. A lateralcondyle 152 extends from the first lateral end 146 to the second lateralend 148 and is formed between the lateral posterior surface 144 and theinternal non-articulating surface 132.

The femoral prosthesis 100 is configured such that the height of themedial condyle 150 is less than the height of the lateral condyle 152,as best shown in FIG. 6(A). The heights of the condyles 150, 152 aremeasured from a line 154 that is tangent to the distal articulatingsurface 136 to the second medial end 142 and second lateral end 148,respectively. In one embodiment, the medial condyle height h_(m) isabout 35.5 mm, but can be in the range of 20 mm to 60 mm, 30 mm to 45mm, 35 mm to 40 mm and the like. In one embodiment the lateral condyleheight h_(l) is about 38.5 mm, but can be in the range of 22 mm to 62mm, 32 mm to 47 mm, 37 mm to 42 mm and the like. The difference inheight between the medial condyle 150 and the lateral condyle 152(h_(l)−h_(m)) can be in the range of about 0.5 mm to 6 mm, about 1 mm to4 mm, about 2 mm to 3 mm and the like. In one embodiment for the femoralprosthesis 100, the difference in height is about three millimeters.This height difference between the medial and lateral condyles 150, 152is also shown in FIG. 7C from a rear, or posterior, view of the femoralprosthesis design 100, as well as in FIG. 6A. This is in contrast to thetraditional femoral prosthesis 10, as shown in FIGS. 7A and 7B, whichhave either equal medial and lateral condyle 50,52 heights(h′_(m)=h′_(l)) or the medial condyle height 50 is greater than that ofthe lateral condyle 52 (h″_(m)>h″_(l), see U.S. Pat. No. 6,770,099 toAndriacchi et al.). In the embodiment, as illustrated in FIG. 6A, theouter articulating surface of the medial femoral condyle in theproximal-posterior is also shifted inwards relative to theproximal-posterior surface of the lateral femoral condyle. In someconfigurations this may be achieved by using a radius r_(m) that is lessthan the radius r_(l), of the lateral condyle 152. In otherconfigurations of the design, the proximal-posterior portions of themedial and lateral femoral condyles may be composed of one or moreradii. These radii (including r_(m) and r_(l)) can have any value, suchas in the range of 2 mm to 70 mm, 10 mm to 40 mm, 25 mm to 30 mm and thelike. In one embodiment of the femoral prosthesis, r_(m) is about 13.5mm and r_(l) is about 20 mm.

The femoral prosthesis design 100 described herein controls excessiveligament tension by having a medial proximal-posterior geometry thatmatches the femoral bone surface 13 even at flexion angles greater than90 degrees, such as further illustrated in FIG. 6B. In addition to themedial femoral condyle 150 height being lowered relative to the lateralfemoral condyle 152 as discussed above, the outer surface of the medialcondyle 150 is shifted inwards from the posterior cartilage surface 28.As shown in FIG. 6B, this may result in the medial femoral condylesurface 150 being pushed inwards i.e. towards the anterior of the kneeapproximately 2.5 mm from the original posterior cartilage surface 28 tomore closely match the bone surface 13 geometry in the posterior region31. The proximal-posterior tip 151 of the medial condyle 150 may also berounded to match the geometry of the bone surface 13. Because the medialfemoral condyle 150 is lowered relative to the lateral femoral condyle152, a portion of the proximal-posterior bone 34 may be removed duringsurgery to avoid impingement of the bone 34 with the tibial component(not shown) of the TKA prosthesis in very deep flexion (greater thanabout 135 degrees). As shown in FIG. 6B, this portion of theproximal-posterior bone 34 that may be removed may be from the medial 22bone surface 13. Removing the proximal-posterior bone 34 and aligningthe second medial end 142 of the medial condyle 150 to match the bonesurface 13 results in the proximal-posterior tip 151 to be lower thanthe native bone surface 13 of the proximal-posterior region 27 of thefemur 12.

FIGS. 8A and 8B illustrate how the femoral prosthesis 100 avoids overstretching of the soft tissue in deep flexion, particularlyoverstretching of the medial soft-tissues. FIG. 8A follows flexion from0 degree to 150 degrees for a knee that has a prosthesis 100, but alsodisplays the profile of the traditional femoral prosthesis 10 forcomparison. The tibial component 56 and medial collateral ligament 58are also shown in FIGS. 8A and 8B. As the knee bends from 0 degreesthrough 90 degrees flexion, both the prosthesis 10 and femoralprostheses 100 allow the medial soft tissue to maintain uniform tension.As shown in FIG. 8A, the medial collateral ligament 58 in a knee withthe prosthesis 100 design does not experience increased tension as theknee is flexed, even for flexion beyond 90 degrees. Also of note in FIG.8A, the removal of the proximal-posterior bone 34 allows the femoralprosthesis 100 to avoid contact with the tibial component 56 of theprosthesis.

However, for a knee with a traditional femoral prosthesis 10, thesituation changes for flexion of the knee beyond 90 degrees. FIG. 8Bshows a comparison of a knee with a traditional femoral prosthesis 10 toa femoral prosthesis 100 as described herein for flexion angles of 135degrees and 150 degrees. As shown in FIG. 8B, for the traditional TKAfemoral prosthesis 10 to flex beyond 90 degrees and maintain uniformligamentous tension, the traditional femoral prosthesis 10 would have todig into or compress the tibial part 56 of the TKA prosthesis. However,the tibial prosthesis 56 is a relatively rigid body, and therefore, theonly way the knee with a traditional prosthesis 10 can flex beyond 90degrees is by stretching the soft tissue in the knee. This stretching ofthe soft tissue in a knee incorporating the traditional femoralprosthesis 10 is demonstrated in FIG. 8B by a change in position of themedial collateral ligament 58. Flexing of the knee with a traditionalfemoral prosthesis 10 becomes progressively more difficult with greaterdegrees of flexion. In fact, after about 110 degrees, the knee istypically too tight to flex any further. In contrast, the femoralprosthesis 100 allows the knee to go into very deep flexion withouthaving to overstretch the soft-tissue, thereby potentially enhancing therange of motion post-TKA surgery. Without overstretching the soft-tissuein the knee, the femoral prosthesis 100 also maintains more symmetricmedial and lateral ligament balance through a full range of motion.

The performance of the femoral prosthesis of FIG. 6A was comparedagainst prior art femoral prosthesis of FIG. 4B using a dynamic kneesimulation software. Both femoral components articulated with identicaltibial and patellar implants, and one full cycle of a deep knee bend (0to 155 degrees flexion and 155 to 0 degree extension) was simulated. Thekinematics of the knee, characterized as motion of medial and lateralfemoral condyles relative to a fixed tibia, were virtually identical forboth the traditional femoral prosthesis and that of the presentdisclosure. However, with the femoral prosthesis of the presentdisclosure the posterior cruciate ligament (PCL) tension wassignificantly reduced for knee flexion above 90 degrees compared to thetraditional femoral prosthesis (reduction in tension of ˜20 percent at120 degrees flexion, FIG. 9). This validates the ability of the femoralprosthesis of the present invention to minimize soft tissue tightness inflexion, particularly for soft tissue on the medial side and/or softtissue attaching to the medial femoral condyle.

The prosthesis described herein can be constructed in various mannersand can be made from one or more materials. The prosthesis can bemachined, cast, forged, molded, or otherwise constructed out of amedical grade, physiologically acceptable material such as a cobaltchromium alloy, a titanium alloy, stainless steel, ceramic, etc. Other,non-limiting examples of materials for the implants include polyolefins,polyethylene, ultra-high molecular weight polyethylene, medium-densitypolyethylene, high-density polyethylene, medium-density polyethylene,highly cross-linked ultra-high molecular weight polyethylene (UHMWPE),and the like.

The present invention provides an improved knee replacement prosthesis,which can facilitate deep knee flexion without creating excessivetension in the ligamentous structure of the knee. A knee replacementprosthesis, such as a TKA prosthesis, is generally composed of a femoralprosthesis that replaces at least a portion of the native femur, atibial prosthesis that replaces at least a portion of the native tibia,and an optional patellar prosthesis that replaces at least a portion ofthe native patella. The present invention describes novel embodiments offemoral prosthesis for knee replacement. A TKA prosthesis is atri-compartmental prosthesis designed to replace all three compartmentsof the knee, namely: the lateral compartment, medial compartment and thepatellofemoral compartment. While, the femoral prosthesis of the presentinvention is described in relation to a tri-compartmental TKAprosthesis, these novel designs are applicable to knee replacementprosthesis involving replacement of one or more compartments of the knee(such as uni-compartment or bi-compartment), including implants with orwithout a tibial or patellar prosthesis.

Although the invention has been described in considerable detail withreference to certain embodiments, one skilled in the art will appreciatethat the present invention can be practiced by other than the describedembodiments, which have been presented for purposes of illustration andnot of limitation. Therefore, the scope of the appended claims shouldnot be limited to the description of the embodiments contained herein.

1. A femoral prosthesis comprising: an internal non-articulatingbone-engaging surface configured to be connected to a resected distalend of a femur; an external articulating surface including a distalarticulating surface, a medial posterior surface having a first medialend and a second medial end, and a lateral posterior surface having afirst lateral end and a second lateral end, wherein the first medial endand the first lateral end are connected to the distal articulatingsurface; a medial condyle extending from the first medial end to thesecond medial end and formed between the medial posterior surface andthe internal non-articulating surface; and a lateral condyle extendingfrom the first lateral end to the second lateral end and formed betweenthe lateral posterior surface and the internal non-articulating surface;wherein a first height of the medial condyle measured from a linetangent to the distal articulating surface to the second medial end isless than a second height of the lateral condyle measured from the linetangent to the distal articulating surface to the second lateral end,and wherein the medial collateral ligament in a knee with the prosthesisdoes not experience increased tension as the knee is flexed, even forflexion of the knee beyond 90 degrees.
 2. The femoral prosthesis ofclaim 1, wherein: removal of a proximal-posterior bone allows thefemoral prosthesis to avoid contact with a tibial component.
 3. Thefemoral prosthesis of claim 1 wherein: the femoral prosthesis allows theknee to go into deep flexion without having to overstretch soft-tissue,thereby enhancing the range of motion of the femoral prosthesis.
 4. Thefemoral prosthesis of claim 1, wherein: the femoral prosthesis maintainssymmetric medial and lateral ligament balance through a full range ofmotion without overstretching the soft-tissue in the knee.
 5. Thefemoral prosthesis of claim 1, wherein: tension in posterior cruciateligament (PCL) is significantly reduced for knee flexion above 90degrees thereby minimizing soft tissue tightness in flexion on themedial side and soft tissue attaching to the medial femoral condyle. 6.The femoral prosthesis of claim 1, wherein a radius of the medialcondyle is less than a radius of the lateral condyle.
 7. The femoralprosthesis of claim 1, wherein the medial condyle further includes aproximal-posterior tip or end, the proximal-posterior tip or end havinga smooth rounded profile.
 8. The femoral prosthesis of claim 1, whereina difference in height between the medial condyle and the lateralcondyle is within the range of about 2 millimeters to about 5millimeters.
 9. The femoral prosthesis of claim 1, wherein the medialposterior surface transitions from a posterior portion in which themedial posterior surface is not shifted inwards relative to the lateralposterior surface to a distal-posterior portion in which the medialposterior surface is shifted inwards relative to the lateral posteriorsurface.
 10. The femoral prosthesis of claim 1, wherein: the medialposterior surface transitions from a posterior portion in which themedial posterior surface is not shifted inwards relative to the lateralposterior surface to a proximal-posterior portion in which the medialposterior surface is shifted inwards relative to the lateral posteriorsurface, such that a posterior thickness of the medial condyle measuredfrom the internal non-articulating bone-engaging surface is equal to aposterior thickness of the lateral condyle.
 11. A method for mounting afemoral prosthesis on a distal end of a femur, the method comprising:providing a femoral prosthesis with a distal articulating surface and aposterior articulating surface; removing a portion of aproximal-posterior bone of the femur; and mounting the femoralprosthesis on the femur, wherein the femoral prosthesis is mounted onthe femur such that the distal articulating surface is approximatelytangent to a cartilage surface on a lateral side of the knee, the distalarticulating surface is approximately perpendicular to a mechanical axisof the femur, and the lateral posterior surface is approximately tangentto a posterior cartilage surface on the lateral side of the knee. 12.The method of claim 11, wherein the portion of the proximal-posteriorbone of the femur is a medial proximal-posterior bone.
 13. The method ofclaim 11, wherein the femoral prosthesis further includes a medialcondyle with a proximal-posterior tip, and the portion of theproximal-posterior bone of the femur that is removed is removed to matchthe proximal-posterior tip of the femoral prosthesis.
 14. The method ofclaim 11, wherein the femoral prosthesis further includes: an internalnon-articulating surface; an external articulating surface including thedistal articulating surface, a medial posterior surface having a firstmedial end and a second medial end, and the lateral posterior surfacehaving a first lateral end and a second lateral end, wherein the firstmedial end and the first lateral end are connected to the distalarticulating surface; a medial condyle extending from the first medialend to the second medial end and formed between the medial posteriorsurface and the internal non-articulating surface; and a lateral condyleextending from the first lateral end to the second lateral end andformed between the lateral posterior surface and the internalnon-articulating surface.
 15. The method of claim 14, wherein a firstheight of the medial condyle measured from a line tangent to the distalarticulating surface to the second medial end is less than a secondheight of the lateral condyle measured from the line tangent to thedistal articulating surface to the second lateral end.
 16. The method ofclaim 15, wherein the medial condyle further includes aproximal-posterior tip, the proximal-posterior tip is shifted inwardsrelative to a proximal-posterior portion of the femur.
 17. The method ofclaim 15, wherein the medial condyle further includes aproximal-posterior tip, the proximal-posterior tip having a smoothrounded profile matching geometry of the native femur.
 18. The method ofclaim 15, wherein the femoral prosthesis comprises a material selectedfrom the group consisting of thermoplastic polymers, thermoset polymers,titanium, titanium alloy, tantalum, cobalt chrome alloy, stainlesssteel, and ceramics.
 19. The method of claim 15, wherein a difference inheight between the medial condyle and the lateral condyle is within therange of about 0.5 millimeters to about 5 millimeters.
 20. The method ofclaim 15, wherein a radius of a proximal-posterior portion of the medialcondyle is less than a radius of a proximal-posterior portion of thelateral condyle.